Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 459

CHAPTER 9
Integration of GPS with Other Sensors
and Network Assistance

J. Blake Bullock
Motorola
Michael Foss
Vehicle Guidance
G. Jeffrey Geier
Motorola
Michael King
General Dynamics

9.1 Overview

In the previous chapters, we have observed that GPS receivers can be thought of as
discrete-time position/velocity sensors with sampling intervals of approximately 1
second. The need to provide continuous navigation between the update periods of
the GPS receiver, during periods of shading of the GPS receiver's antenna, and
through periods of interference is the impetus for integrating GPS with various addi-
tional sensors. The most popular are inertial sensors, but the list also includes
dopplerometers (Doppler velocity/altimeters), altimeters, speedometers, and odom-
eters, to name a few. The method most widely used for this integration is the
Kalman filter. The Kalman filter is an estimator. It estimates the instantaneous state
of a linear system perturbed by Gaussian white noise. One of the key attributes of
the Kalman filter is that it provides a means of inferring information by the use of
indirect measurements. It does not have to read control variable(s) directly, but it
can read an indirect measurement (including associated noise) and estimate the con-
trol variable(s). In GPS applications, the control variables, as we will see later in this
chapter, are position, velocity, and possible attitude errors. The indirect

measurements are GPS measurements.
In addition to integration with other sensors, it can also be extremely beneficial
to integrate a GPS sensor within a communications network. For example, many
cellular handsets now include embedded GPS engines to locate the user in the event
of an emergency or to support a wide variety of location-based services. These
handsets are often used indoors or in other areas where the GPS signals are so highly
attenuated that demodulation of the GPS navigation data by the handset is not pos-

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